The present invention relates to a terminal control device for a reference station in a time division multiple access (TDMA) satellite communication system. More particularly, the present invention is concerned with a terminal control device usable for regular synchronization control and initial acquisition control which are performed by a reference station on TDMA terminals, the latter control preceding the former control.
As well known in the art, a TDMA communication system includes a reference station and a plurality of earth stations, or terminals, and uses a TDMA frame which is divided into time slots which are assigned one to each earth station. Each earth station transmits a burst of its own within a particular one of the time slots which is assigned thereto. The reference station, therefore, has to constantly supervise the terminals so that the bursts sent from the various terminals may each be confined in the time slot assigned to the associated terminal. Such a control, which is generally referred to as a transmit burst synchronization control, is one of those basic functions which are fulfilled by a reference station.
In an initial state before a certain earth station, or terminal, sends a burst, the precise timing at which the burst should be sent is unknown and, hence, initial acquisition control is needed in order to determine that timing. Specifically, in a TDMA satellite communication system, a reference station starts controlling a certain terminal after performing initial acquisition control prior to regular synchronization control. During the initial acquisition control, the reference station provides a wide exclusive acquisition window for this particular control in a receive frame thereof so as to supply the intended terminal with satellite position information, reference station position information, and timing information derived from the position of the intended terminal and other factors. In response, the terminal sends a burst of its own as instructed by the timing information.
Confirming reception of the burst from the terminal in the acquisition window, the reference station prepares new timing information based on a difference between the time base associated with the burst reception and the time base associated with the regular synchronization control, the new timing information being sent to the terminal. At the same time, the reference station cancels the acquisition window in the receive TDMA frame and, instead, sets up a window in a regular synchronization control position. In this condition, when the terminal sends a burst responsive to the timing information, the reference station receives it in the regular synchronization window.
Here, the prerequisite is that the exclusive window for the initial acquisition control in the TDMA frame be wider than a certain one partly because error in the satellite position and fixed delay particular to a terminal are unknown. Initial acquisition control systems known in the art may generally be classified into two types with respect to the treatment of the window, i.e., a so-called parallel acquisition type and a so-called sequential acquisition type. The parallel acquisition type system is such that TDMA terminals which are controlled by a single reference station have individual acquisition windows in a TDMA frame. The sequential acquisition type system is such that the terminals share a common acquisition window in a TDMA frame.
The prior art initial acquisition system, whether it be parallel or sequential, has problems left unsolved. Specifically, the problem with the parallel acquisition scheme is that the reference station has to define acquisition windows in a TDMA frame which are assigned one to each of the controlled terminals. Such multiple acquisition windows limit the number of terminals which may be controlled by a common reference station as well as the length of a burst which each terminals may send, thereby degrading the TDMA frame utilization efficiency to a critical degree. The sequential acquisition scheme, on the other hand, enhances efficient use of a TDMA frame because a reference station needs only a single acquisition window in a TDMA frame with no regard to the number of controlled terminals. However, due to the single acquisition window, the reference station is incapable of starting on initial acquisition control for a plurality of terminals at the same time. For this reason, when the satellite communication link is cut off due to some failure in the TDMA communication network, the sequential acquisition scheme requires a far longer period of time than the parallel acquisition scheme to recover the link by enabling all the associated terminals. In addition, the period of time necessary for the link to recover is proportional to the number of terminals which was controlled by the reference station. Such is contradictory to the advantage particular to the sequential acquisition system, i.e. the capability for controlling a great number of terminals with enhanced TDMA frame utilization efficiency.